The present invention relates to a power failure protector circuit for preventing spurious output signals from an electronic circuit when the power supply for the electronic circuit drops below its normal operating level. More particularly, the present invention provides a means for preventing spurious output signals from an electronic circuit when its power supply is turned off or drops below some predetermined level required for proper operation of the circuit.
Electronic circuits with active circuit elements such as transistors and vacuum tubes may produce spurious output signals when the power supply which provides the bias voltages for the active elements is below its normal operating range. The spurious output signals may occur when the power supply is interrupted, the term interrupted being used herein to collectively define normal turn-off or an abnormal condition which results in either a drop of the power supply voltage below its normal operating range or an intermittent or permanent loss of power.
Power supply interruptions occur frequently in some environments. For example, in farm machinery such as combine harvesters, electronic circuits are used for various control purposes and these circuits are frequently located up to 25 feet from the power supply, the connection being made through circuits which may include multiple cable connectors and relay contacts. The circuit connections through the cable connectors and relay contacts are not always reliable and are sometimes intermittent. Furthermore, the power supply is frequently the battery used in the ignition system for the combine harvester and its output voltage may drop when the engine is started or restarted. All of these factors cause variations in the bias voltages supplied to the active elements within the electronic circuits and, as is well known, such variations in the bias voltages may cause unreliable operation of the individual active elements in the circuits thereby resulting in spurious or unwanted output signals from the circuits.